The present invention relates generally to the pressurizing and mixing of gases in a closed chamber, and more particularly to the pressurizing and mixing of a low permeability gas with air in a tennis ball center just prior to the joining of two halves of the center together to form a complete pressurized ball.
The pressurizing of tennis ball centers with a low permeability gas such as sulfur hexafluoride or perfluoropropane is set forth in the above mentioned related applications. The advantage of using such low permeability gases is that they do not permeate out of the tennis balls as readily as air, and consequently they result in tennis balls that have longer playing lives.
However, in order for the low permeability gases to be effective as a commercially feasible way of increasing the playing life of tennis balls, the gas must be distributed fairly evenly throughout the mold. Otherwise, only a portion of the tennis balls produced will receive enough low permeability gas to effectively increase their playing lives. Also, customers receiving a good set of balls on one occasion are likely to be disappointed on a future occasion when they receive a set of balls that do not have as much low permeability gas.
A particular problem with sulfur hexafluoride and other low permeability gases is that, being heavier than air, they tend to stratify in the air already in the mold and collect in the mold cavities nearest the point in the mold where they are introduced. Eventually, the gases do mix by diffusion, but this process takes a long time, as much as 10 or more minutes, to achieve a satisfactory gas distribution throughout all the mold cavities.
It is not desirable to solve this problem by premixing the low permeability gas with the air and introduce the mixture into an evacuated mold, because when the mold is evacuated, the residual air trapped between the ball halves and the surfaces of the mold cavities tends to force the ball halves out of the cavities. It is then impossible for these ball halves to be properly pressurized or joined to each other by the closing of the mold sections after the pressurization step. Another method tried for mixing the gases in the mold is bumping the mold after pressurizing it. However, such bumping has not proven effective in reducing the mixing time to an appreciable extent. Also, bumping the mold can break the seal around the peripheries of the mold sections, which results in lower ball center pressures as well as a loss of valuable gas.